1. Field of the Invention
The present invention concerns variable angle vane arrays in axial fluid flow machines. It is particularly, but not exclusively, concerned with variable area nozzle vane arrays suitable for use in power turbines forming part of gas turbine engines of the kind utilised in industrial and marine environments, for example the propulsion of ships.
2. Description of the Prior Art
Due to the need to optimise performance of power turbines in gas turbine engines, a nozzle vane array which directs a working fluid onto a power turbine should have the capability of varying the its nozzle area. This can be achieved by pivoting the vanes in unison about axes extending radially of the turbine, so varying the vanes' angle with respect to the flow of fluid therepast. By this means, the total throat area of the nozzle can be varied between maximum and minimum scheduled values during normal operation of the engine.
In mechanisms for achieving pivoting of the vanes, it is usual to connect each vane via respective levers to a common actuating or unison ring surrounding the turbine casing, so that when the ring is rotated about the turbine axis, the vanes pivot in unison to either increase or decrease the nozzle throat area.
In one known type of mechanism, the lever arms comprise gear segments, one end of the lever arm including the gear circle centre, which is fixed to a respective vane, the other end comprising the toothed rim of the gear segment. The gear segment teeth mesh with a toothed unison ring to enable simultaneous pivoting of the vanes.
In an emergency, such as turbine shaft breakage, leading to overspeed of the power turbine, it is highly desirable to have the capability of effecting substantially total obturation (ie shut-down of the nozzle. This action substantially prevents the working fluid impinging on the turbine rotor blades, thereby avoiding freewheeling runaway of the rotating parts of the turbine with consequent severe damage.
Vane actuating levers in the form of gear segments have a drawback, in that if the number of vanes in the array is such that they are closely spaced around the circumference of the turbine casing, the gear segments cannot be made large enough to remain in toothed engagement with the ring while pivoting sufficiently to substantially close the throat of the stage, without interfering or overlapping with each other.